Superconductive cable system

ABSTRACT

A cryogenic cable system for power transmission applications has a hollow conductor of a material which is superconductive at the freezing temperature of hydrogen and which is maintained at that temperature by providing the interior of the hollow conductor with a mixture of solid and liquid hydrogen.

United States Patent 1 Feb. 15, 1972 Minnich 154] SUPERCONDUCTIVE CABLESYSTEM [72] Inventor: Stephen H. Minnich, Schenectady, NY. [73]Assignee: General Electric Company [22] Filed: Mar. 19, 1969 [21] Appl.No.: 809,481

[52] US. Cl ..l74/15, 62/55, 174/26, 335/216 [51] Int. Cl. ..ll0lb 7/34[58] Field ofSearch ..174/15 C, 15, D16. 6, 36,102, 174/24, 25, 26, 27,34; 335/216; 62/45, 55

[56] References Cited UNITED STATES PATENTS 3,455,117 7/1969 Prelowski..62/55 X 3,292,016 12/1966 Kafka ...174/15 X 3,432,783 3/1969 Brittonet al.... .....335/216 3,461,218 8/1969 Buchhold 174/15 FOREIGN PATENTSOR APPLICATIONS 1,061,922 3/1967 Great Britain ..174/l5 1,510,13812/1967 France ..l74/SC OTHER PUBLICATIONS Cook, G. A. & Dwyer, R. F.,Fluid H \'zlr0gen S/llS/Ir1 Review Advances in Cryogenic Engineering.Plenum V01. 11 p. 202-206 (TP-480-A3-C.2)

Primary Examiner-Lewis H. Myers Assistant Examiner-A. T. GrimleyAttorney-Paul A. Frank, John F. Ahem, Julius J. Zaskalicky, Frank L.Neuhauser, Oscar B. Waddell and Joseph B. Forman ABSTRACT A cryogeniccable system for power transmission applications has a hollow conductorof a material which is superconductive at the freezing temperature ofhydrogen and which is maintained at that temperature by providing theinterior of the hollow conductor with a mixture of solid and liquidhydrogen.

5 Claims, 2 Drawing Figures FATENTEDFEB 15 I972 ma /W u 4H bSUPERCONDUCTIV E CABLE SYSTEM The present invention relates to powertransmission cable systems for operation at cryogenic temperatures.

Cryogenically cooled cable is being considered for the transmission ofpower at high voltages. Such cable has the promise of increasing thepower transmission capacity of a transmission system of given size. Suchcable would be particularly useful for underground applications wheresize is a factor. For such underground applications both superconductiveand resistive, that is ultrahigh conductivity, systems are beingconsidered. In both types of systems the losses which must be removed tomaintain the desired mode of operation may be categorized as conductor,dielectric, shield and heat leak losses. The dielectric and heat leaklosses are, in general, the same for both types of systems. However, theconductor and shield losses for superconductive systems can be at leasta factor of 10 less than for resistive systems. Also, such conductor andshield losses for superconductive systems are considerably less than thedielectric and heat leak losses. In resistive systems, operated underalternating current conditions, the losses are due to eddy currentsinduced into the conductors and to the resistance of the conductors. Insuperconductive systems operation under alternating conditions such eddycurrent and resistive losses do not occur; however, other alternatingcurrent losses occur which, in general, are considerably less than theeddy current and the resistive losses in resistive systems.

Most superconductive systems require operation in the vicinity of 4 K.as practical superconductors require cooling to that temperature toobtain superconductivity and reasonable currents. Accordingly, whilesuperconductive systems are possible in which the electrical powerlosses in the cable are reduced, at the same time, aside from practicalproblems, considerably more refrigeration power input to achieve the lowtemperature is required than would be saved by the elimination of theeddy current and other losses characteristic of resistive systems atliquid hydrogen temperatures. The present invention is directed to asuperconductive power transmission system in which the above describedproblems are eliminated.

Accordingly, it is an object of the present inventionto provide a powertransmission cable system using superconductors for the transmission ofpower at cryogenic temperatures comparable to those forhigh-conductivity metals such as copper or aluminum used in a resistivesystem, with comparable refrigeration efficiencies but withsubstantially lower loss than such conventional metals.

Another object of the present invention is the provision of a powertransmission cable system for controlling the temperature of thesuperconductor at the lowest possible value consistent with liquidhydrogen cooling.

A further object of the present invention is to provide a powertransmission cable system in which a liquid refrigerant is used forcooling the cable using go and return streams in which the g and returnstreams flow in opposite directions yet which do not require thermalisolation to achieve good cooling along the entire length of the system.

In carrying out the invention in one illustrative embodiment, there isprovided a hollow tube of material which is superconductive at thefreezing temperature of hydrogen. A mixture of solid and liquidhydrogen, i.e., slush hydrogen, is provided substantially filling theinterior space of the hollow tube to maintain the tube at the freezingtemperature of hydrogen.

The novel features which are believed to be characteristic of thepresent invention are set forth in the appended claims. The inventionitself, however, together with further objects and advantages thereofmay be understood by reference to the following description taken inconnection with the accompanying drawings in which:

FIG. 1 is a perspective view in partial section of an embodiment of acable in accordance with the present invention;

FIG. 2 is a cross-sectional view of a three-phase power transmissioncable incorporating the cable structure of FIG. 1.

Referring now to FIG. 1, there is shown an illustrative embodiment of acryogenic cable 10 in accordance with the present invention having aninner former or hollow insulating tube 11 surrounded by a conductivelayer 12 tubular in form and an outer layer 13 of high-voltageinsulating material. The outer surface of the layer 13 is encased in aconductive shield 14. The former 11 could be a plastic material such asnylon or polyethylene. The conductive layer 12 is made of asuperconductive material, for example, niobium tin, Nb -,Sn, which is asuperconductor at the freezing temperature of hydrogen. i.e.. 14 Kelvin.Niobium tin has a transition temperature of 18 Kelvin. Niobium tin has ahigh critical current density at 14 Kelvin, i.e., at least of the orderof 100,000 amperes per square centimeter at the field intensitiesencountered in the present application. One method of applying suchconductor to form the conductive layer 12 would be as a helical wrap ofthin tape of niobium tin such as now produced for commercial sale bysuch companies as the General Electric Company of Schenectady, NY. Suchtape consists of niobium tin on a backing of copper. The thickness ofniobium required for the present application would be a few thousandthsof an inch. Copper bonded to the niobium tin tape as in the GeneralElect'ric product would provide conduction during transient overloadconditions should the critical current of niobium tin be exceeded. Forminimizing eddy current losses in the copper during normal operation, itshould be placed on the inside of the niobium tin conducting layer. Ifdesired, a thin stainless steel strip, for example a layerone-thousandth of an inch thick, could be bonded to the outer surface ofthe niobium tin tape to provide mechanical protection thereto. If thestainless steel layer is thin, negligible eddy current losses will occurin it. Use of the superconductor in other forms such as a sprayedcoating on a solid tube is also possible. Other superconductors withsufficiently high critical temperature may as well be usedv A tapedlayer of electrical insulation 13 consisting of cellulose paper orsynthetic paper tape impregnated with hydrogen fluid is provided aboutthe conductive layer 12. The shield 14 is provided to eliminate strayfields around the cable which would produce losses in a cryogenic pipefor containing the cable as will be explained below in connection withFIG. 2.

The shield 14 may be composed of a bimetallic layer of niobium tin andcopper tape similar to that used on the conductor 12. In the case of theshield the copper layer is to the outside where the magnetic fluxdensity is zero.

Referring now to FIG. 2, there is shown a composite three- I phase powertransmission cable 20, each of the individual cables 21, 22 and 23 ofwhich incorporate the structure of the cable described in FIG. 1. Thethree cables 21, 22 and 23 are housed in cryogenic pipe 24 having aninner casing 25 of a suitable and conventional material such as steeland an outer casing 26 of a suitable and conventional material such assteel concentric with the inner casing 25. The space between the innercasing 25 and the outer casing 26 is evacuated and may be filled withthermal insulation such as super insulation consisting of a plurality ofcylindrical layers of reflective material and insulating material. Aplurality of spacers 28 are provided between the inner casing 25 and theouter casing 26 to maintain the integrity of the space therebetween. Theshields of cables 21, 22 and 23 are cooperatively connected in themanner described and claimed in a copending patent application, Ser. No.539,089, filed Mar. 31, 1966, now US. Pat. No. 3,461,218, and assignedto the assignee of the present invention to eliminate stray magneticfields which would otherwise produce losses in the steel pipes. If theshields of the conductors 21, 22 and 23 are electrically connectedperiodically along the length thereof, shield currents equal andopposite to the load currents will flow, whereby the currents in theshield add vectorially to zero and the net magnetic field producedoutside the shields is zero.

In the operation of a composite cable, slush hydrogen is passed in onedirection through the opening in the center of the hollow formers ofeach of the three cables 21, 22 and 23 and is passed in the oppositedirection through the space between the shields of the individual cables21, 22 and 23 and the inner pipe 25 of the cryogenic envelop 24. In aclosed system the cryogenic fluid is pumped from one end of the cable 20to the other and returned. Refrigerators may be located at selectedintervals along the power transmission line 20 if desired. Slushhydrogen flowing in the bore of the three conductors 21, 22 and 23 maybe removed at selected intervals and rerefrigerated to restore the solidwhich was melted during its passage through the bores of the conductors.ln the case of a similar cooling circuit for resistive cables, the goand return streams have a certain temperature rise. The temperaturedifference between such go and return streams involves an exchange ofheat therebetween. The thermal resistance of good electrical insulationis not enough to prevent such heat exchange. Such counterflow heatexchange impairs the efficacy of the resistive system. In the case of asuperconductor system, since the two streams are maintained at the sametemperature, namely the temperature of slush hydrogen, 14 Kelvin, thecounterflow heat exchange is precluded thus simplifying the coolingcircuits and the resultant cable system configuration. Hydrogen has afreezing point of l4 Kelvin. The heat of fusion of solid hydrogen is58.5 joules per gram and its density is 0.087 g./cm. Liquid hydrogen hasa specific heat of 7.3

joules per gram-degree Kelvin near 14 and its density is 0.075

tolerated, it is of great importance in a superconductive cable wheretemperature rises at 1 to 2 Kelvin might be tolerated, but which wouldbe undesirable.

It is apparent that the slush-hydrogen cooled superconductor combinationapplies also to direct current cable systems. In the direct currentcase, the cable system described would be even more attractive than inthe alternating current case,

since superconductor losses and dielectric losses are absent,

and the only loss to be absorbed by the refrigeration system is the heatleak loss. In addition, for the direct current core, the currentcarrying conductor could be smaller, since it would not be necessary towrap the superconductor on a large diameter former, which is done in thealternating current case to reduce surface flux density.

While the invention has been described in specific embodiments, it willbe appreciated that many modifications may be made by those skilled inthe art and l intendby the appended claims to cover all suchmodifications and changes as fall within the true spirit and scope ofthe invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A cryogenic cable comprising a hollow tube of material which issuperconductive at the freezing temperature of hydrogen, and a mixtureof solid and liquid hydrogen substantially filling the interior space ofsaid hollow tube.

2. The combination of claim 1 in which 'said superconductive material isNb sn. v

3. The combination of claim 1 in which said hollow tube ofsuperconductive material is in the form of strips of material helicallywound on a hollow former of insulating material.

4. A cryogenic cable comprising an inner hollow tube of material whichis superconductive at the temperature of slush hydrogen, an outer hollowtube of material which is superconductive at the temperature of slushhydrogen, said outer hollow tube being of larger diameter than said onetube and surrounding said inner tube in insulating relationshiptherewith to provide an electromagnetic shield therefor, slush hydrogensubstantially filling the interior space of said inner hollow tube.

5. The combination of claim 4 which includes an enclosure for saidhollow tubes and in which slush hydrogen substantially fills the spacebetween said outer hollow tube and said enclosure for said hollow tubes.

2. The combination of claim 1 in which said superconductive material isNb3Sn.
 3. The combination of claim 1 in which said hollow tube ofsuperconductive material is in the form of strips of material helicallywound on a hollow former of insulating material.
 4. A cryogenic cablecomprising an inner hollow tube of material which is superconductive atthe temperature of slush hydrogen, an outer hollow tube of materialwhich is superconductive at the temperature of slush hydrogen, saidouter hollow tube being of larger diameter than said one tube andsurrounding said inner tube in insulating relationship therewith toprovide an electromagnetic shield therefor, slush hydrogen substantiallyfilling the interior space of said inner hollow tube.
 5. The combinationof claim 4 which includes an enclosure for said hollow tubes and inwhich slush hydrogen substantially fills the space between said outerhollow tube and said enclosure for said hollow tubes.